Polyimide resins have excellent heat resistance along with high mechanical strength, flame retardance, electrical insulating property and the like so that they have conventionally been used in the fields of electric and electronic equipment, space and aircraft equipment and transportation equipment. Their utility is expected to expand from now on to fields where heat resistance is required. Some polyimide resins having excellent properties have already been developed.
Polyimide resins which have heretofore been developed, however, have poor processability despite their excellent heat resistance. Further, resins of the type developed, for example, for improved processability are inferior in heat resistance and solvent resistance. Thus, the conventional polyimide resins have both merits and demerits.
Described specifically, polyimide resins ("Kapton" and "Vespel", trade names; products of E.I. du Pont de Nemours & Co., Inc.) which are described in J. Polym. Sci. Macromol, Rev., 11, 161 (1976) and J. Elast. Plast., 7, 285 (1975) and have a basic skeleton structure represented by the following formula (5); ##STR2## have no distinct glass transition temperature and have excellent heat resistance. It is, however, difficult to process these resins when they are used as molding materials. They have to be processed using techniques such as sinter molding. When used as materials for electric or electronic parts, they are accompanied by the drawback that the dimensional stability, insulation property and solder resistance are adversely affected.
A polyether imide resin ("ULTEM", trade name; product of General Electric Co., Ltd.) having a basic skeleton represented by the following formula (6): ##STR3## has excellent processability. It has, however, a glass transition point as low as 217.degree. C. and thus has inferior heat-resistance. It is also unsatisfactory from the viewpoint of solvent resistance, for it is soluble in halogenated hydrocarbons such as methylene chloride.
With a view toward overcoming these drawbacks, the present applicant developed polyimide resins having recurring units represented by the following formula (1): ##STR4## wherein X represents a direct bond or a group selected from the group consisting of a divalent hydrocarbon group having 1-10 carbon atoms, a hexa-fluorinated isopropylidene group, a carbonyl group, a thio group, a sulfonyl group and an oxo group; Y.sub.1, Y.sub.2, Y.sub.3 and Y.sub.4 individually represent a group selected from the group consisting of a hydrogen atom, a lower alkyl group having 1-9 carbon atoms, a lower alkoxy group having 1-9 carbon atoms, a chlorine atom and a bromine atom; and R.sub.1 represents a tetravalent group having 2-27 carbon atoms, said tetravalent group being selected from the group consisting of aliphatic groups, monocyclic aliphatic groups, monocyclic aromatic groups, fused polycyclic aromatic groups and non-fused polycyclic aromatic groups with aromatic rings bonded together directly or via a crosslinking member and have already filed patent applications thereon (Japanese Patent Laid-Open Nos. 236858/1987 and 253655/1987).
The polyimide resins described above have been used widely for their excellent properties such as thermal and electrical properties. These resins have, however, a low crystallization velocity and their molded products obtained by injection molding, for example, at a mold temperature of 50.degree.-250.degree. C. is generally amorphous. To employ them as crystallized products, heat treatment is needed such as the method disclosed in Japanese Patent Laid-Open No. 110538/1989. This method, however, is conducted under long-hour and high-temperature conditions. At a temperature of 270.degree. C., for example, heat treatment as long as 12 hours or longer is required. In addition, the molded polyimide resin products are deformed upon this heat treatment, leading to the problem of a substantial, dimensional change. On the other hand, heat treatment in a short time requires a high temperature so that deterioration of the resin is promoted.
To improve the drawbacks of such molded polyimide resin products, the present applicant also found that blending of a thermotropic liquid crystalline polymer with the polyimide resin is effective, resulting in an application for patent (Japanese Patent Laid-Open No. 175373/1992). The resin composition so obtained is excellent in heat resistance, toughness, processability and the like so that expansion of its use is under way to such fields wherein a metal or ceramic has heretofore been considered indispensable. Molded products obtained from this resin composition are, however, accompanied by the drawback that they have substantial anisotropy. By the addition of the liquid crystalline polymer, the crystallization velocity of the polyimide resin is effectively increased, so that the conditions for liquid crystallization are relaxed. The resin composition so obtained, however, involves the problems that molded products tend to develop warpage after heat treatment and molded products undergo peeling at portions corresponding to gates owing to dispersion of the liquid crystalline polymer into a layer in the vicinity of the gates.
A resin composition obtained by adding a polyether ketone to the above polyimide resin is also disclosed in Japanese Patent Laid-Open No. 175373/1992. This resin composition, however, is inferior in heat stability and flowability at high temperatures and therefore has drawbacks in molding conditions.
As has been described above, the conventional polyimide resin compositions are still not satisfactory.